Electronic piano feedback reduction



March 25, 1969 A. C. IPPOLITO ELECTRONIC PIANO FEEDBACK REDUCTION Filed March 31, 1965 Sheet of 3 '00 REED EXCURSION 61424 c/ ry CHANGE 1114856 REED Ex x5 ION cmflcrry elm/v65 REED BTU/i510 JOMEW/MT P457" PICKUP PLATES CAPH CIT) CHKMIGF- REED EXCURSION IVE/IRS P/C'KI/P PL/I TF C/IPH c/Ty elm/v65- 1 0w AMPLITUDE REED 3552 59 I I /cw ]6 i J8 P flfilL/F/ER Z r L :F I ILQL l 4a #46 54 52 54 55 Z 53 mw 2 1% 54 I 52 5B 4 a Z '54 54 (d) I raw Max'ch 25, 1969 A. C. IPPOLITO ELECTRONIC PIANO FEEDBACK REDUCTION Filed March 31, 1965 Sheet 3 of 5 R550 Breaks/0N 0 aqpnc/ry C'l/fl/Vf REED EXCURSION 60M: WHAT ins?- PICKUP P4192":

CAPAC/ Ty (l/ANGE- low 4MPL lTl/DE March 25, 1969 A, .PPOUTO 3,425,122

ELECTRONIC PIANO FEEDBACK REDUCTION Filed March 31, 1965 sheet 3 of 5 (a) REED Excuas/m/ C JZ CAP/Ic/ry CW/MIGE w 6 114K6 5 REED EXCUKSION 4%; 54$

546 Z .5 9 4 5Z A cnPnc/ry elm/v R550 P4556 ONE PIC UP Pl; TE 545 K 46 & CAP/zany CHANGE Z550 6021/25 0 E4 R5 IC 535 556 556 PLATES CA fine/7y (WA/v65 10W 4MPL/ r005 J/ZZ Z/Zfif firzfkoy flj ipolzfo' 5%; 7%, zw/rw W United States Patent US. Cl. 841.14 4 Claims ABSTRACT OF THE DISCLOSURE An electronic piano utilizing cantilevered, vibratile reeds as the tone generating elements. The reeds are in electrical capacitive relation with a comb-shaped pickup, vibrating in the slots in the pickup to vary the capacity between the reeds and the pickup, which variation in capacity is electrically amplified and converted in a loudspeaker into audible piano-like tones. The positional relation of each reed to the pickup is such that, at low amplitudes of vibration, the variation in capacity contains a high percentage of second harmonic as compared with the fundamental frequency of vibration of the reed, whereby there is little fundamental frequency energy available that might be fed back from the loudspeaker to the reed, whereby undesirably to sustain reed rvibration.

This invention relates to the art of electronic music production, and more particularly to improved features in an electronic piano.

The conventional string piano is generally recognized as being the most popular solo instrument, and is widely used both in concert and in homes. The conventional piano does have certain drawbacks, which in given circumstances may be quite substantial. In the best instruments, namely grand pianos, the instrument is quite large and requires a substantial amount of floor space. Even in upright pianos, the instrument is not small. Any conventional piano weighs at least several hundred pounds, and therefore is not readily portable. In addition it is impossible for a student to practice a piano without having the sound therefrom audible over a substantial area. To correct these and other deficiencies, an electronic piano has been developed and commercialized utilizing reeds which are set in vibration by more or less conventional piano actions and which vibrate in electrostatic capacitive relation with a fixed pickup. The capacity change between the reeds and pickups is applied to either a tube type or transistor type electronic amplifier, and the amplified tones are converted into audible sound by a loudspeaker, or by earphones for individual practice. The piano so made is relatively small and light in weight, and can be carried from place to place as a portable instrument.

As will be understood, undue amplitude of reed vibration shortens the service life of reeds as compared with relatively small amplitudes of vibration. It will be apparent that the smaller the amplitude of vibration, the smaller the electrical signal generated. This is readily compensated for simply by increasing the gain in the amplifier. However, increasing the gain in the amplifier increases the seriousness of an inherent difliculty, namely electroacoustic feedback.

In the electronic piano as mentioned heretofore, all of the parts are mounted in a rather small case. A certain.

amount of energy from the loudspeaker is transmitted back to the reeds through the case and through the air in the case. This is obviously undesirable as it will tend to cause a reed to remain in vibration a longer period than is desired, and in an extreme case can even result in runaway of the reed, i.e. increasing amplitude of vibration until the reed would destroy itself.

Short of such an extreme case, feed-back energy can keep one or more reeds in sustained vibration, as contrasted with the desired tonal decay of a string piano.

Accordingly, it is an object of the present invention to eliminate the deleterious effects of electro-mechanical feedback in an electronic piano utilizing vibrating reeds as tone generators.

It is a concomitant object of the present invention to produce from an electronic piano tones more closely resembling those of a conventional stringed piano.

More specifically, it is an object of this invention to provide a reed and pickup structure in an electronic piano which for relatively large reed excursion produces a great deal of second harmonic relative to the fundamental tone, thereby simultaneously diminishing the effects of electromechanical feedback and producing a more piano-like tone.

Other and further objects and advantages of the present invention will be apparent from the following description 'when taken in connection with the accompany drawings wherein:

FIG. 1 is a somewhat schematic diagram illustrating the feedback problem;

FIG. 2 is a side view of a reed and pickup substantially eliminating the problem;

FIG. 3 is a perspective view of one of a plurality of reeds and a common pickup therefor;

FIG. 4 is a fragmentary top view showing a plurality of reeds and the common pickup;

FIGS. 5 (a)5 (e) comprise diagrams illustrating wave shapes of the electrical oscillations generated by the reed of FIGS. 2-5;

FIG. 6 is a side view similar to FIG. 2 showing a modification of the invention;

FIGS. 7(a)-7(e) is a diagram of wave shapes somewhat similar to FIGS. 5(a)-5(e), but corresponding to the reed and pickup of FIG. 6.

FIG. 8 is a side view similar to FIGS. 2 and -6 showing a further modification of the invention; and

FIGS. 9(a)-9(e) are wave shapes similar to FIGS. 5 and 7 but corresponding to the reed and pickup of FIG. 8.

Although the present disclosure is complete in and of itself, those who wish to become more versed in the background and in the complete electronic piano are referred to any of several United States Letters Patent issued to C. W. Anderson, including the following: 2,881,651, 2,909,093, 2,949,052, 2,949,053, 2,952,179, and 2,974,555.

Referring now in greater particularity to the drawings, and first to FIG. 1, there will be seen the tone generating or producing portions of an electronic piano comprising a vibratile reed 10 disposed adjacent a pickup 12, the latter being grounded. The reed is connected by a suitable electric connection means indicated at 14 to an amplifier 16, preferably employing transistors or vacuum tubes. The amplifier preferably has volume control means for playing the piano at different volume levels, and the amplifier is connected to a loudspeaker 18. All of the foregoing parts, and also the keys and actions for setting the reeds into vibration are enclosed within a case indicated by the dashed lines 20. As has heretofore been mentioned, there is a certain amount of feedback from the loudspeaker 18 to the reed 10, both through the case 20 and through the air in the case. Such feedback is indicated at 22.

Reference now should be had to FIGS. 2-4 when there will be seen a vibratile reed 24 disposed adjacent a pickup 26 and in electrostatic capacitive relation therewith. The reed is set into vibration by a piano action including a hammer 28. The reeds are made of electrically conductive material, spring steel being one satisfactory example. A plurality of reeds mounted in side-by-side relation, is seen in FIG. 4, being individually secured to the top of an insulating member 30, as by screws 32, the insulating memher being supported on the top of a relatively massive metallic member 34. All of the reeds are wired in parallel to the amplifier 16.

The pickup 26 is comb shaped as viewed from the top (FIG. 4), comprising a plurality of teeth 36 projecting between the ends of the cantilevered reeds 24, the reeds fitting in slots 38 between the teeth. As will be seen in FIGS. 2 and 3, the pickup is somewhat in the shape of a deep channel, having relatively wide upper and lower, spaced apart parallel plates or walls or flanges 40 spaced apart by a relatively narrow, integral web 42. The upper and lower plates are identical to one another, having overlying fingers 36 and recesses 38. Preferably the space between the upper and lower plates is filled with an insulating 1 plastic material 44 (preferably a glass base phenolic). In

FIG. 3 the plastic material has been removed from between the forward portions of the plates 40 for clarity of illustration, but is seen between the third fingers 36, measured from the left foreground to the right background. The insulating material also has been left out of FIG. 2 f r simplicity of illustration.

The position of the reed 24 in each instance is precisely related to the position of the plates 40 of the pickup, and the amplitude of vibration is likewise so related. Specifically, as seen in FIG. 2 most clearly, the reed 24 is precisely half way between the two plates 40. Its amplitude of vibration, as indicated in dashed lines above and below the upper and lower plates respectively, is such that the reed is capable of vibrating beyond both of these plates.

It is to be understood that the reeds at the bass end of the scale are the largest, and that the reeds progressively diminish in size up toward the treble end of the scale. The amplitude of vibration of the reeds at the treble end, and even in the mid range is not very great, and the problems of feedback are minimal. It is at the bass end that problems are encountered, and therefore the principles of this invention are directed primarily to the bass reeds.

Attention now is directed to FIG. 5, wherein various wave forms are shown. In FIG. 5(a), the reed excursion is illustrated. As will be appreciated, except for some initial distortion due to the percussive impulsing of the reed, the reed vibrates with a sine wave shape 46, having alternate positive half cycles 48 and negative half cycles 50. In FIG. 5(b), the capacity between the reed and pickup, and hence the voltage generated, is illustrated. The time base is the same. It will be observed that with the reed at its central rest position, the residual capacity as indicated at 52 is rather low. When the reed is first struck with a substantial blow, it is impulsed beyond the upper pickup plate 40. Thus, the capacity quickly reaches a maximum at 54 when the reed passes the upper plate 40. The capacity then drops to a minimum at 56 while the reed is beyond the plate 40 and this minimum may or may not be the same as that at 52, depending on the physical relation of the parts. The minimum is, of course, at the point where the reed has its maximum excursion, although there is a rather shallow valley, as will be appreciated, since the capacity drops to a rather low value when a reed has passed the pickup plate. Subsequently, when the reed descends past the pickup plate, another peak of capacity value is produced at 58, and the capacity rapidly drops to its initial minimum value, again indicated as 52, this being 180 of reed excitation or vibration after its initial rest positon. Subsequently, as the reed passes the bottom pickup plate 40 there is another peak of maximum capacity value, again labeled as 54 due to its correspondence in value and also in time relation, but delayed 180. Likewise, there is another minimum capacity value, again indicated at 56 when the reed has reached its farthest downward excursion. Thereafter, as the reed comes up past the bottom plate 40, there is another maximum peak 58, following which the capacity drops off to its initial minimum 52 as the reed reaches centralized position. Thus, it will be seen that the 4 are two pips produced for each half cycle of reed vibration. Thus, a very strong fourth harmonic is generated with the reed vibrating at substantially its maximum excursion. As will be appreciated, there is still a substantial fundamental component, but this is less than the fourth harmonic.

As the reed dies down in amplitude of excursion, there will come a time when the maximum reed excursion is just to and very slightly beyond the pickup plates 40. This condition is illustrated in FIG. 5(0), where it will be seen that the first pip 54 has moved to the right, toward the position relative to reed excitation. Since the reed does not swing so far past the upper pickup plate, the minimum 56 is raised quite substantially. Due to the same consideration of passage of the reed only slightly past the plate, the first pip 58 has been moved to the left, again toward the 90 position of reed excitation. The minima 52 remain as heretofore. The same condition prevails during the negative half cycle, the peaks or pips 54 and 58 moving closer to the 270 position, and the minimum 56 again being raised.

The pips 54 and 58 as described above are primarily fourth harmonic. However, it will be appreciated that from a Fourier analysis that there are other components present, including the fundamental. When the reed excursion decays to the point where the reeds just reach or do not quite reach the pickup plates, then the maximum or peak 54 shifts over to the 90 position as shown in FIG. 5 (d). The peak 58 loses its separate identity, being merged with the peak 54, and there is no valley or minimum 56. The minimum 52 remains the same as heretofore.

Subsequently, as reed excursion decays still further, the wave form will remain much the same as in FIG. 5(d) but the amplitude of the peak 54 will be diminished, and the peak will broaden somewhat, see FIG. 5 (e).

From the foregoing, it will be appreciated that at no time does a wave comprise primarily a strong fundamental. Generally speaking, it is only the fundamental that is a problem in feedback, since the reed does not tend to respond in vibration to fedback energy of harmonics. Also, it will be observed that the harmonic structure changes during the period of free decadent vibration of the reed after it has been percussively excited, and this causes the resulting tone to sound more like that of a stringed piano.

A modification of the invention is shown in FIG. 6. In FIG. 6, and also in FIG. 7, similar numerals are utilized with the sufiix a to identify like parts. The essential difference, as seen in FIG. 6, is that the pickup 2611 does not comprise two plates, but rather comprises a single plate 60. This plate is of the comb shape heretofore described, and it lies somewhat above the reed 24a. The reed during its upward excursion passes beyond the plate 60, as is shown in dotted lines, and also swings farther away from the plate during its downward excursion.

With reference now to FIG. 7, it will be seen that the reed excursion again is a sine wave, plotted exactly the same as in FIG. 5(a). The capacity change through the first 180 of reed excursion, as plotted opposite FIG. 7(b), is the same as it was in FIG. 5(b). However, it should be noted for the sake of accuracy that the ampli tude at the zero time point and also at 180, as indicated at 52(a), is not truly a minimum, even though it is the capacity at rest position. This is due to the departure, as seen from 180 to 360 in FIG. 7(b) from the previous wave form. Since there is no second plate, the capacity continues to decrease past 180, and reaches a minimum which is substantially zero somewhat before the 270 position (the maximum downward reed excursion), and remains substantially at zero to somewhat past the 270 position, rising again to the zero degree position 52(a) at 360. This rather broad, substantially zero minimum is indicated at 62.

When the reed passes somewhat past the pickup plate 60, the resulting wave form is as shown in FIG. 7(a). The

first 180 of vibration produces a wave form similar to that shown in FIG. 5(c). However, when the reed swings downward from its rest position it again produces a substantially zero minimum 62, generally of somewhat lesser width.

When the reed then diminishes in amplitude of vibration to the position where it nears or just reaches the pockup plate, the peak 54a shifts to the 90 position, just as it did in FIG. 5 (d). However, on the negative half cycle the capacity again reaches substantially zero, this time being even closer to a negative peak at the 270 position and of less width.

The wave form of FIG. 7(d) is substantially repeated in 7(e), although the peak 54a is reduced in height due to the fact that the reed does not vibrate as close to the pickup plate.

In FIGS. 7(b) and 7(0) it will be seen that a second harmonic is produced as compared with the fourth harmonic in FIGS. 5(b) and (c). In FIGS. 7(d) and (e) it is essentially the fundamental that is produced as opposed to the second harmonic in FIGS. 5 (d) and (e). It will be appreciated that the amplitude of the fundamental in FIGS. 7(d) and (e) is sufficiently low that feedback problems are of no importance. It further will be appreciated that even though it is primarily a fundamental that is seen in FIGS. 7(d) and (e), there will be a certain amount of harmonics, since the wave is not a sine wave. It again will be seen that the harmonic structure shifts as the reed decays in amplitude of vibration, and hence a more natural piano-like sound is produced. Thus, in both embodiments of the invention as heretofore shown and described a very significant portion of the total energy of the reed is devoted to producing second or fourth harmonics during the time of greatest reed excursion, and hence of greatest energy. As the energy is dissipated so that problems of feedback are of little or no significance,

the harmonics become of relatively less importance. Not only does the construction herein disclosed result in no substantial feedback of fundamental, but it produces a more piano-like tone due to the shifting harmonic structure with decay of vibrational amplitude.

A preferred concept of the invention is illustrated in FIG. 8. In FIG. 8, and also in FIG. 9, the numerals again are utilized to identify like parts, this time with the addition of the suflix b. The embodiment in FIG. 8 is generally similar to that in FIG. 2 in that two pickup plates 40b are provided. These plates are connected by a conductive spacer 42b, and are held together by a bolt 64 passing through the plates and spacer. The pickup plates 40b are of comb shape as heretofore described. The essential difference in the present embodiment of the inventionis that one of the plates is of uniform thickness, while the other plate, shown as the upper plate is cut down to provide a thin projection 66 comprising the tooth portion of the plate. A corresponding portion of the second plate is not cut down, and is indicated at 68 as being twice or more the thickness of the cut-down plate projection 66.

At maximum reed excursion, the reed may pass both pickup plates in the projecting portions 66 and 68. Upon somewhat lesser excursion, the reed will pass the thin plate portion 66, but will not pass the thick portion 68. Furthermore, it will be appreciated that during maximium excursion, the reed passes beyond the thin pickup plate projection 66 before it passes completely beyond the thicker plate projection 68.

The wave shapes are illustrated in FIG. 9, and the reed excursion again will be seen to be a sine wave, plotted exactly the same as in FIG. 5(a). The capacity change at maximum reed excursion is generally similar to what it was in FIG. 5, although it will be observed that the first two peaks 54b are spaced farther apart than the second two peaks 54b, since the reed passes the thin plate projection 66 sooner than it does the thick plate projection 68, as noted above.

Upon lesser amplitude of vibration, as illustrated in FIG. 9(0), the peaks 54b remain during the upward half cycle of reed vibration, but during the lower half cycle of reed vibration, the reed does not pass the plate projection 68, and hence the peaks of maximum capacity merge together as a single peak indicated at 54b at 270. Subsequently, upon lesser vibrational amplitude, as illustrated in FIG. 9(d), the wave shape is generally the same as in FIGS. 5 (d), and the same is true also with regard to low amplitude reed excursion, the capacity corresponding thereto being illustrated at FIG. 9(e).

Thus it will be observed that a fourth harmonic is produced in FIG. 9(b), and a second harmonic in FIGS. 9(d) and 9(e), generally similar to FIG. 5, but having somewhat different overall harmonic structure due to the difference in wave shape, particularly in FIG. 9(b). The greatest departure is in FIG. 9(0), wherein it will be observed that a strong third harmonic is produced.

Bearing in mind that none of the wave shapes is a sine wave, insofar as the capacity change is concerned, it will be apparent from a Fourier analysis that the fundamental and many other harmonics are present, but that the strongest or greatest capacity change is the fourth, third, or second harmonic, as noted.

Again, the presence of substantial energy in the harmonics during greatest reed excursion minimizes problems of feedback, tending to prevent undue sustaining or hanging-on of vibration. The shifting harmonic content with decay of vibration produces perhaps an even more natural piano-like sound and is produced with the two embodiments of the invention shown and described earlier.

The specific examples of the invention as herein shown and described will be understood as being illustrative only. Various changes in structure will no doubt occur to those skilled in the art, and will be understood as forming a part of the present invention insofar as they fall within the spirit and scope of the appended claims.

The invention is claimed as follows:

1. Tone generating means for an electronic piano including a case, an amplifier mounted in said case, and a loudspeaker mounted in said case, said case and air in said case tending to feed acoustic energy from said loudspeaker back to said tone generating means, said tone generating means being mounted in said case and comprising a vibratile reed and a pickup in electrostatic capacitive relation thereto, means electrically connecting said reed and said pickup to said amplifier, means mounting said reed in cantilevered position in a predetermined plane, and impulse means for percussively exciting said reed into decadent free vibration substantially perpendicular to said plane, said pickup including a pair of electrodes spaced normally from said plane on opposite sides thereof, said electrodes being of different thicknesses, said reed at different amplitudes of vibration passing beyond both of said electrodes, passing beyond the thinner of the two electrodes and not beyond the other, and not passing beyond either electrode.

2. Tone generating means for an electronic piano including a case, an amplifier mounted in said case, and a loudspeaker mounted in said case, said case and air in said case tending to feed acoustic energy from said loudspeaker back to said tone generating means, said tone generating means comprising a plurality of vibratile reeds and a pickup in electrostatic capacitive relation thereto in said case, means electrically connecting said reeds and said pickup to said amplifier, means mounting said reeds in parallelism in cantilevered position in a predetermined common plane, and impulse means for selectively percussively exciting said reeds into decadent free vibration substantially perpendicular to said plane, said pickup including a common electrode spaced normally of said plane from said reeds and comprising a substantially flat, comb-shaped plate parallel to said reed plane and having a plurality of alternate fingers and recesses, said reeds vibrating through said recesses, each of said reeds upon initial excitation vibrating beyond said electrode plate and subsequently vibrating short of said electrode plate, whereby upon small reed vibration the capacity change between a reel and said pickup has a high second harmonic content relative to the frequency of reed vibration to minimize energy fed back from said loudspeaker to said reeds at the fundamental frequency of said reeds.

3. Tone generating means as set forth in claim 2 wherein said pickup common electrode includes a second combshaped plate parallel to said reed plane and spaced 0n the opposite side of said reed plane from the first-mentioned plate, said reed being disposed midway between said electrode plates and at different amplitudes of vibration passing beyond both plates, passing into but not beyond both plates, and vibrating short of both plates.

4. Tone generating means as set forth in claim 3 UNITED STATES PATENTS 2,656,755 10/1953 Miessner 84--1.l4 2,974,555 3/1961 Andersen 84-1.14 3,041,909 7/1962 'Miessner 84--1.14 3,033,363 6/1962 Miessner 841.l4

ARTHUR GAUSS, Primary Examiner.

B. P. DAVIS, Assistant Examiner.

US. Cl. X.R. 84-l.l5 

